Daniel is 3 times as old as Ben. Fifteen years ago, Daniel was 8 times as old as Ben. How old is Daniel now?
We can use the given information to write down two equations that describe the ages of Daniel and Ben. Let Daniel's current age be $d$ and Ben's current age be $b$ The information in the first sentence can be expressed in the following equation: $d = 3b$ Fifteen years ago, Daniel was $d - 15$ years old, and Ben was $b - 15$ years old. The information in the second sentence can be expressed in the following equation: $d - 15 = 8(b - 15)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $d$ , it might be easiest to solve our first equation for $b$ and substitute it into our second equation. Solving our first equation for $b$ , we get: $b = d / 3$ . Substituting this into our second equation, we get: $d - 15 = 8($ $(d / 3)$ $- 15)$ which combines the information about $d$ from both of our original equations. Simplifying the right side of this equation, we get: $d - 15 = \dfrac{8}{3} d - 120$ Solving for $d$ , we get: $\dfrac{5}{3} d = 105$ $d = \dfrac{3}{5} \cdot 105 = 63$.